1. Field of the Invention
The present invention relates to a transmission in which plural gears are relatively rotatably supported on rotational shafts and in which a desired shift stage can be achieved by selectively connecting any one of the plural gears to the corresponding one of the rotational shafts by means of a corresponding clutch mechanism. The present invention also relates to a method of up-shifting for the transmission.
2. Description of the Related Art
Japanese Patent No. 3838494 discloses a transmission for a bicycle with the following characteristic features. The transmission includes three transmission shafts disposed in parallel to one another. Plural gears corresponding respectively to various shift stages are fixed on a second transmission shaft located in the middle of the three transmission shafts. Plural gears corresponding respectively to the shift stages are relatively rotatably supported on each of first and third transmission shafts located respectively on the both sides of the second transmission shaft. The gears on the second transmission shaft are meshed with the gears on the first and the third transmission shafts. Each of the first and the third transmission shafts is formed to have a hollow structure. Either a ratchet one-way mechanism or a ball one-way mechanism is disposed between each of the first and the third transmission shafts and each of the gears supported on the outer circumference thereof. The one-way mechanisms are selectively activated by moving, in the axial direction, clutch operation elements disposed in the insides of the first and the third transmission shafts. As a consequence, chosen ones of the gears are selectively connected to the first and the third transmission shafts, so that the desired one of the shift stages is achieved.
In addition, Japanese Utility Model Application Publication No. 61-39871 discloses a transmission with the following characteristic features. In this transmission, drive gears are fixed on an input shaft, while driven gears are relatively rotatably supported on an output shaft. The drive gears are meshed with their respective driven gears. Installation holes penetrate the output shaft in the radial direction thereof, and steel balls are installed in the installation holes. A shift lever is disposed inside the input shaft so as to be movable in the axial direction thereof. Some of the steel balls are moved with the shift lever in the radial direction of the output shaft, so as to be engaged with a fitting groove formed in the inner circumferential surface of the corresponding driven gear. As a consequence, one of the driven gears is connected to the output shaft, so that the desired one of the shift stages is achieved.
In the transmission disclosed in Japanese Patent No. 3838494, however, while the one-way mechanism for the achieved shift stage is engaging, the one-way mechanisms for the other shift stages are slipping. A problem thus arises in that the friction due to the slipping of the one-way mechanisms reduces the transmission efficiency. In addition, there is a following problem because the one-way mechanisms can slip in any moment. While the transmission can be used in the case of bicycles in which the driving force is not transmitted during the deceleration but is transmitted only during the acceleration, the transmission, however, cannot be used for vehicles, such as automobiles, in which the driving force has to be transmitted during the deceleration for the engine brake and regenerative brake.
The transmission disclosed in Japanese Utility Model Application Publication No. 61-39871 has the following problems. Since the driving force is transmitted through the point contact of the steel balls, the surface of each steel ball may be worn or deformed. In addition, even when the one-way mechanism is slipping, the steel balls are pressed onto the inner circumferential surface of the driven gear by the centrifugal force. As a consequence, friction acts between the steel balls and the driven gear, thereby decreasing the transmission efficiency.
Moreover, both of the transmissions disclosed by Japanese Patent No. 3838494 and by Japanese Utility Model Application Publication No. 61-39871 have the following problem. During an up-shifting operation from a lower shift stage to a higher shift stage, there is a time lag between the disengaging of the gear-set for the prior shift stage and the completion of the engaging of the gear-set for the subsequent shift stage. The transmission of the driving force from the engine to the drive wheel is temporarily discontinued during the time lag, so that the acceleration performance is reduced. The mechanism for the reduction is as follows. The engagement of the gear-set for the subsequent shift stage requires the connection of the relevant gear to the rotational shaft by means of the clutch mechanism. The clutch mechanism, however, does not work until the phase of the gear to be connected becomes a predetermined state. Accordingly, the transmission of the driving force becomes impossible when the gear-set for the prior shift stage is disengaged, and continues to be impossible until the phase of the relevant gear of the gear-set for the subsequent shift stage becomes the predetermined state.